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techniques.Typically these studies were carried out by preparing a 10-mM
solution of the relevant calix[41pyrrole in dichloromethanew/, and adding
increasing substoichiometric quantities of the tetrabutylammomum salt of
a given anion in dichloromethane^/.. As a genera! rule, the pyrrole NH
and CH proton resonances were found to shift upon addition of anions, a
finding that is indicative of an interaction between the macrocycle and
(he added anionic guest species. As a representative example of (his kind
of experiment, the titration curve of compound 1 with fluoride anions is
reproduced in Figure 2a. In this case and others. Job's method of
continuous variation was used to verify that the complexes formed in
solution possessed a
1 : 1 ealixpyrrole:anion stoichiometry (Figure 2b). Stability-constant
determinations were then made using (he EQNMR least-squares-fitting
procedure.^ The findings reveal that both compounds 1 and 2 not only are
effective anion-binding agents in solution, bu( also selective, showinn a
Sessler and Gale
0.005 0.01 0.015 0.02
Concentration of Fluoride (mol dm'3)
0.6 + [F
Figure 2. ^ NMR anion coordination studies: (a) 'H NMR titration curve
for com pound 1 and fluoride anions in dichloromethane-c/2 and (b) molar
ratio (|ob) plot of compound 1 with fluoride anions indicating a 1:1
Table 1. Stability Constants (M ') for Compounds 1 and 2 with Anionic
Substrates13 in Dichloromethane-d7 at 298 Ê
Anion Ka( M- ')
Compound 1 2
Fluoride h 17170 (±900) 3600 ( ± 395)
Chloride 350 (±5.5) 117 (±4.0)
Bromide 10 (±0.5) N.D.
Iodide < 10 N.D.
Dihydrogenphosphate 97 (±3.9) < 10
Hydrogensulfate < 10 N.D.
Source: Data from Gale, P. A.; Sessler, J. L.; Krai V.: Lynch. V. J. Am.
Chem. Sue. 1996. 1/8, 5140.
" Anions were added as 0.1 M dichloromethane-d2 solutions of their
tetrabutylammonium salts to lOmM solutions of the receptor in dichloro-
methane-^2 with concentration changes being accounted for by EQNMR. In
determining the stability constants, the possible effects of ion pairing
(if any) were ignored.
h Tetrabutylammonium fluoride was added as the trihydrate.
N.D.: Not determined.
marked preference for fluoride over other anionic guests such as chloride
and dihydrogenphosphate (Table 1 ). 's Crystals of the chloride complex
of meso-octamethylca-lixpyrrole 1 and the fluoride complex of meso-
tetra-spirocyclohexylcalixpyrrole 2 were obtained by slow evaporation
of a dichloromethane solution of the ligands containing an excess of the
respective tetrabutylammonium halide salt. X-ray crystal structural
analysis revealed that both complexes adopt a cone conformation such that
the four NH protons can hydrogen bond to the halide anion (Figure 3). The
nitrogen-to-chloride distances in l-Cl- are in the range of 3.264(7)-
3.331(7) A, while in 2-F" they are 2.790(2) A (Figure 3b) (the four
pyrrole groups are equivalent by symmetry). As a result, in these two
complexes the chloride and fluoride anions are located 2.319(3) and
1.499(3) A above the N4 root mean square planes of calixpyrroles 1 and
2. respectively. These
Figure 3. X-ray crystal structures of (a) the chloride complex of meso-
octamethylcalixpyrrole 1 -n-Bu4NCI-CH2Cl2 and (b) the fluoride complex
of meso-tetraspirocyclohexylcalixpyrrole 2-n-Bu4NF. Counter cations
and solvent molecules are omitted for clarity. Crystal structures
originally published in Gale, P. A.; Sessler, J. L.; Krai, V.; Lynch, V.
j. Am. Chem. Soc. 1996, 118, 5140. Diagram produced using data from the
Cambridge Crystallographic Database.
45 / Calixpyrroles: Novel Anion and Neutral Substrate Receptors
Methyl group resonance
.6 1.5 1.4
Recorded at 500 MHz in CD2CI2
Figure 4. Variable temperature 'H NMR spectra (in dichloromethane-d2) of
compound 1 in the presence of three equivalents of fluoride anions. The
meso-methyl groups become inequivalent as the temperature is lowered.
results are thus consistent with the anion being more tightly bound in
the solid state.
Variable temperature 'H NMR studies were carried out on a
dichloromethane-(/i solution of 1. In the absence of fluoride anions
there was no significant change in (he 'H NMR spectrum of 1 as the
temperature was lowered to 193 K. However, in the presence of three
equivalents of tetrabutylammonium fluoride, the wie.w-methyl resonance
splits into two as the temperature is lowered (Figure 4). This splitting
is presumably due to the calixpyrrole-fluoride complex adopting a cone